linux-kernel - Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

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Date:   Tue, 10 May 2022 09:32:13 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     torvalds@...ux-foundation.org, damien.lemoal@...nsource.wdc.com,
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Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

On Mon, May 09, 2022 at 06:28:17PM -0400, Theodore Ts'o wrote:
> Oh, one other problem with DEPT --- it's SLOW --- the overhead is
> enormous.  Using kvm-xfstests[1] running "kvm-xfstests smoke", here
> are some sample times:

Yes, right. DEPT has never been optimized. It rather turns on
CONFIG_LOCKDEP and even CONFIG_PROVE_LOCKING when CONFIG_DEPT gets on
because of porting issue. I have no choice but to rely on those to
develop DEPT out of tree. Of course, that's what I don't like.

Plus, for now, I'm focusing on removing false positives. Once it's
considered settled down, I will work on performance optimizaition. But
it should still keep relying on Lockdep CONFIGs and adding additional
overhead on it until DEPT can be developed in the tree.

> 			LOCKDEP		DEPT
> Time to first test	49 seconds	602 seconds
> ext4/001      		2 s		22 s
> ext4/003		2 s		8 s
> ext4/005		0 s		7 s
> ext4/020		1 s		8 s
> ext4/021		11 s		17 s
> ext4/023		0 s		83 s
> generic/001		4 s		76 s
> generic/002		0 s		11 s
> generic/003		10 s		19 s
> 
> There are some large variations; in some cases, some xfstests take 10x
> as much time or more to run.  In fact, when I first started the
> kvm-xfstests run with DEPT, I thought something had hung and that
> tests would never start.  (In fact, with gce-xfstests the default
> watchdog "something has gone terribly wrong with the kexec" had fired,
> and I didn't get any test results using gce-xfstests at all.  If DEPT
> goes in without any optimizations, I'm going to have to adjust the
> watchdogs timers for gce-xfstests.)

Thank you for informing it. I will go for the optimization as well.

> The bottom line is that at the moment, between the false positives,
> and the significant overhead imposed by DEPT, I would suggest that if
> DEPT ever does go in, that it should be possible to disable DEPT and
> only use the existing CONFIG_PROVE_LOCKING version of LOCKDEP, just
> because DEPT is S - L - O - W.
> 
> [1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md
> 
> 						- Ted
> 
> P.S.  Darrick and I both have disabled using LOCKDEP by default
> because it slows down ext4 -g auto testing by a factor 2, and xfs -g
> auto testing by a factor of 3.  So the fact that DEPT is a factor of
> 2x to 10x or more slower than LOCKDEP when running various xfstests
> tests should be a real concern.

DEPT is tracking way more objects than Lockdep so it's inevitable to be
slower, but let me try to make it have the similar performance to
Lockdep.

	Byungchul